The present invention relates to a gate turn-off thyristor (hereinafter abbreviated to GTO). More particularly, the present invention pertains to a junction structure which is suitable for improving the interrupting capacity of a buried gate type GTO without increasing the on-state voltage.
GTO is one type of thyristor that can be turned on or off in response to the gate signal and can control large electric power as compared with transistors. To improve the interrupting capacity of GTO, it is effective practice to reduce the transverse resistance of the cathode base layer and thereby to enable the gate turn-off current to be readily drawn out, and to increase the reverse breakdown voltage between the gate and the cathode and thereby to allow the power supply voltage of the gate circuit to be increased. One example of the conventional junction structure which enables both the above-described requirements to be satisfied simultaneously is a so-called buried-gate structure such as those disclosed in the specifications of Japanese Pat. Publication No. 10984/1980 and Japanese Pat. Laid-Open No. 11578/1976. In this type of conventional junction structure, the cathode base layer is provided therein with a partial semiconductor layer which has the same conductivity type as that of the cathode base layer and which has a relatively low resistance, that is, a heavily doped semiconductor layer, and this semiconductor layer is connected to the gate terminal, from which the gate turn-off current is drawn out. This structure enables the transverse resistance of the cathode base layer to be substantially reduced. Since this conventional structure allows a lowering of the impurity concentration of the cathode base layer around the low-resistance layer, it is possible to increase the reverse breakdown voltage between the gate and the cathode at the same time.
Hitherto, the above-described low-resistance layer has the structure in which a multiplicity of elongated strip-shaped slits are provided as described in the material EDD-82-66 furnished at a study meeting of the electrical society. The cathode gate layer extends through the slits in the low-resistance layer to define the passage of main current in an on-state. This is because carriers injected into the cathode base layer from the cathode emitter layer disappear in the low-resistance layer.
In order to further improve the interrupting capacity in the conventional low-resistance layer structure, it is necessary to narrow the width of the slits, but if the slits are narrowed, the conducting area in an on-state is reduced, and the on-state voltage is increased undesirably. The conducting area can be enlarged by increasing the number of slits. However, an increase in the number of slits reduces the width of a portion of the low-resistance layer which is present between each pair of adjacent slits, resulting in an increase in the gate drawing out resistance and a lowering of the interrupting capacity. Thus, the conventional low-resistance layer structure involves the problem that it is difficult to improve the interrupting capacity without increasing the on-state voltage.